Ἀλλήλοις θ᾽ ὁμιλεῖν, ὡς τοὺς μὲν φίλους ἐχθροὺς μὴ ποιῆσαι, τοὺς δ᾽ ἐχθροὺς φίλους ἐργάσασθαι. ἴδιόν τε μηδὲν ἡγεῖσθαι.
We ought so to behave to one another as to avoid making enemies of our friends, and at the same time to make friends of our enemies.
–As quoted in Diogenes Laërtius, Lives and Opinions of Eminent Philosophers, “Pythagoras”, Sect. 23, as translated in Dictionary of Quotations (1906) by Thomas Benfield Harbottle, p. 320; courtesy of Wikiquote.